An image forming apparatus adopting a conventional photoelectronic technique is designed so that, as shown in FIG. 14, a charger 102, an exposing device 103, a developing device 104, a transfer charger 105, a cleaning device 106, and a discharger lamp 107 are provided around a photosensitive drum 101 disposed at center.
In such an image forming apparatus, the photosensitive drum 101 is first charged to a predetermined potential and a predetermined polarity by the charger 102 (charging step), then a surface of the photosensitive drum 101 is exposed by a laser light or the like projected thereto by the exposing device 103. Here, the surface of the photosensitive drum 101 comes to have areas which are exposed thereby undergoing lowering of potentials, and areas which are not exposed thereby not undergoing lowering of potentials. With such potential differences in areas, an electrostatic latent image is formed on the surface of the photosensitive drum 101 (exposing step).
The electrostatic latent image, developed by the developing device 104, becomes a toner image (developing step). The toner image is transferred by the transfer charger 105 onto a transfer material (paper) 110 supplied from a paper feed section, not shown (transferring step).
Thereafter, the transfer material 110 on which the toner image has been transferred is subjected to a fixing step conducted by a fixing section, now shown, and discharged to outside the apparatus.
On the other hand, toner remaining on a surface of the photosensitive drum 101 after the transferring step is removed from the surface of the photosensitive drum 101 by the cleaning device 106 (cleaning step). Further, remaining charges on the surface of the photosensitive drum 101 are removed by the discharger lamp 107 (discharging step), and the operation immediately shifts to the next image forming step.
The removal of remaining charges in the photosensitive drum 101 by the discharger lamp 107, conducted after completion of the transferring step, is an indispensable step. Without this discharging step, uniformness of a charged state is degraded, and this causes exposure memory, or a phenomenon of lowering of surface potentials at only specific portions.
Particularly, in the case where a charger of a contact type is used as the charger 102, a discharging step is not conducted while recharging by the charger 102 is repeatedly carried out, thereby causing the potential of the photosensitive drum 101 to rise. In some cases, this may cause insulation breakdown of the photosensitive drum 101. In the case where a charger of a non-contact type such as scorotron is used as the charger 102, it much less likely causes insulation breakdown of the photosensitive drum 101, but the surface potential of the photosensitive drum 101 gradually rises as the recharging is repeatedly carried out, and this is not desirable for the photosensitive drum 101.
Provision of discharging means to be used exclusively for discharge, such as the discharging lamp 107, however, hinders efforts for further reduction of the size and costs of the image forming apparatus. Therefore, techniques for discharge without discharging means used exclusively for discharging has conventionally been developed.
The Japanese Publication for Laid-Open Patent Application No. 3883/1994 (Tokukaihei 6-3883 [Publication Date: Jan. 14, 1994]) discloses a technique (technique 1 in which after a part of a photosensitive drum is discharged by a laser light emitted from an exposing device, a laser light with a normal image forming exposure power is further projected thereon so that a potential on the photosensitive drum is lowered.
In this technique, after a normal image forming step, a voltage is once applied to a main charger to raise the surface potential of the photosensitive drum, then, the surface of the photosensitive drum is reversely charged by a transfer charger so that the surface potential becomes low, and thereafter, a development bias voltage is turned off. After this turning-off of the development bias voltage, a laser light is projected from an exposing device while its power is controlled, and a laser light with a normal image forming exposure power for image exposure is projected thereon so that the surface potential of the photosensitive drum is lowered to such a potential that no fog occurs (about 100V to 300V as a potential difference between the potential of the photosensitive drum and the development bias voltage). Thus the photosensitive drum surface is electrically cleaned.
Furthermore, the Japanese Publication for Laid-Open Patent Application No. 80870/1997 (Tokukaihei 9-80870 [Issue Date: Mar. 28, 1997]) discloses a technique (technique 2) in which a discharging operation is started in response to a detection signal of detecting means.
In this technique, in an image forming apparatus using a contact-type charger, after an AC voltage applied to a charging member is turned off upon completion of the image forming operation, a transfer material discharge sensor as detecting means detects a front edge or a rear edge of a transfer material and outputs a detection signal for discharge of the photosensitive drum before power-off of the apparatus. Based on the detection signal, an AC voltage is applied to the charging member again, and memory of the photosensitive drum is removed.
In this technique, particularly, it is implied that lamp-use exposing means for uniformly exposing the photosensitive drum can be used in the place of the charging means.
The foregoing techniques 1 and 2 provide only schemes applicable during normal operations, and do not provide measures to cope with irregular events. For example, in the case where a trouble during an image forming operation interrupts the image forming process before the discharge of the photosensitive drum is completed, the photosensitive drum is consequently left having a high surface potential. Therefore, this causes the following problem to arise: during an attempt to correct the trouble until finally the next image forming operation starts, the apparatus is left in a state in which toner adhesion or carrier adhesion tends to occur.
Furthermore, in the discharging step, irrespective of cases, the case where the discharge lamp 107 is used, or the case where light used for writing such as laser emitted by the exposing device 103 is used without provision of the discharge lamp 107, the surface potential of the photosensitive drum 101 is made to sharply drop either by keeping the discharge lamp 107 turned on always or by projecting light always. Therefore, a problem arises in that the photosensitive drum 101 is excessively exposed, thereby causing the operational efficiency of the apparatus to lower.